Crystal structure refinement, low- and high-temperature X-ray diffraction and Mössbauer spectroscopy study of the oxoborate ludwigite from the Iten'yurginskoe deposit.

This paper reports an investigation of the chemistry, crystal structure refinement and thermal behavior (80-1650 K) of ludwigite from the Iten'yurginskoe deposit (Eastern Chukotka, Russia). Its chemical composition was determined by electron microprobe analysis, giving an empirical formula (Mg 1.70 Fe 2+ 0.29 Mn 0.01 ) Σ2.00 (Fe 3+ 0.90 Al 0.08 Mg 0.02 ) Σ1.00 O 2 (BO 3 ). A refinement of the crystal structure from single-crystal X-ray diffraction data (SCXRD) was provided for the first time for ludwigite from this deposit (R = 0.047). The structure can be described as a framework composed of [MO 6 ] n- octahedra and isolated [BO 3 ] 3- triangles located in triangular interstices of the framework. Based on a comprehensive analysis of SCXRD and Mössbauer spectroscopy data, the M1 site is occupied by Mg, M2 and M3 by Mg and Fe 2+ , M4 by Fe 3+ , Mg and Al. There are also oxo-centered [O4M 4 ] n+ and [O2M 5 ] n+ polyhedra building up a framework with the [BO 3 ] 3- triangles located in its hexagonal interstices. No indications of magnetic ordering are found in the temperature range investigated. The Fe 2+ → Fe 3+ oxidation occurs above 600 K, and is accompanied by a decrease of the unit-cell parameters and subsequent incomplete solid-phase decomposition with the formation of hematite, warwickite and magnetite. The mineral melts at temperatures above 1582 K. The thermal expansion of ludwigite is slightly anisotropic, which is explained by a dense packing of the [MO 6 ] n- octahedra as well as a virtually perpendicular orientation of the oxo-centered double chains to each other. At room temperature, maximum expansion is along the c axis (α c = 9.1 × 10 -6  K -1 ) and minimum expansion is in the ab plane (α a = 8.6 × 10 -6 , α b = 7.6 × 10 -6  K -1 ), which is due to the preferred orientation of the [BO 3 ] 3- triangles. A comparison of the thermal behavior of three oxoborates of the ludwigite group, namely azoproite (Mg,Fe 2+ ) 2 (Fe 3+ ,Ti,Mg,Al)O 2 (BO 3 ), vonsenite (Fe 2+ ,Mg) 2 (Fe 3+ ,Mn 2+ ,Sn,Al)O 2 (BO 3 ) and ludwigite (Mg , Fe 2+ , Mn) 2 (Fe 3+ , Al , Mg)O 2 (BO 3 ), is provided.